Recent Stories

The Photosynthetic Ceiling

The Photosynthetic Ceiling

The concept of a photosynthetic ceiling was first introduced in Jared Diamonds book Collapse. Though Diamond certainly did not pioneer the idea, as far as I can tell he was the first to use the easily digestible term photosynthetic ceiling, often referred to in other studies as human appropriation of terrestrial net primary production (HANPP). Such terms highlight Diamonds genius as a popularizer of difficult scientific concepts.

In an interview last year. Diamond explains his notion of the photosynthetic ceiling:

One might think, in fact I thought 30 years ago, theres no limit on the energy that can be fixed by sunlight because the sun is there and theres nothing that we humans will do to diminish the energy falling on the earth from the sun. But the limit is that the energy falling on earth from the sun falls on a certain area of earth and how much plants can grow per year on an acre depends on temperature and rainfall. At present we humans are utilising something like 70% of all the energy from the sun falling on the earth converted by photosynthesis, we are using about 70% of that energy for photosynthesis of human-related plants namely our crops and our golf courses and then caving over natural habitats with cement. So thats 70%. But if the worlds consumption doubles because China and India achieve their goals then that would mean that we are using more than 100% of the energy from sunlight which we cant do. So the photosynthetic ceiling simply means humans appropriating energy from the sun fixed in photosynthesis, all for our purposes leaving no sun energy over for natural habitats including coral reefs and rainforests and eucalyptus and natural vegetation.

Yet the peer reviewed article Diamond references in his own book paints a much more nuanced picture. According to Imhoff et al. humans are currently using between 14 and 26 percent of total net primary production (NPP) for our consumption; namely, we are using photosynthesis to produce and consume vegetal food, meat, milk, eggs, paper, fibre, and wood (both as fuel and material).

What makes their analysis particularly compelling is its focus on geographic variability of HANPP. Their maps clearly demonstrate the unequal use and consumption of photosynthetic material, with large swaths of India and China clearly reliant on imports to sustain a landscape that is currently at capacity. A further breakdown of consumption patterns of HANPP by per capita numbers reveals both expected and unexpected results: North America is first, followed by South America (!), Europe, Africa, and Asia. But South America only uses about 6% of its total photosynthetic capacity, while Europe and Asia average in somewhere around 70%.

The implications of this research go far beyond unequal resource distribution and stressed ecosystem services. To start, the research does not take into account fossil fuel consumption, which is a key point to emphasize. Per capita fossil fuel consumption follows a very similar pattern to HANPP, the difference being that we are dealing with a non-renewable resource now widely considered responsible for potentially catastrophic changes in our global climate.

The concept of the photosynthetic ceiling calls into question the ecological feasibility of renewables as an alternative source of energy. If North America is currently using about 23% of all available sunlight for human consumption, what would that percentage look like when we take into account the solar energy stored in a tank of gas or a lump of coal? And what are the ecological implications of converting to an ethanol economy, or a switchgrass economy, or a solar economy? It is possible that the wholesale destruction of our terrestrial ecosystems as a result of our ravenous hunger for renewable energy will exacerbate global ecosystem collapse, thereby accelerating mass species extinction and destroying the complex web of ecosystem services that act as the foundation of our civilization.

If all this seems like too much gloom and doom for the average urbanitewell, I dont think this added complexity makes renewables an untenable proposition. But, renewable energy must be accompanied by some other very serious measures. To start, weve got to find ways to increase biological efficiency; we need to get more from less. Agricultural waste should become a relic of a forgotten era; we should become like the mythological Native Americans of the mid-Western tall grass prairies, using every single piece of the buffalo lest a single tooth or drop of blood go to waste. Paul Stamets takes this concept to new heights in his new book Mycelium Running. The use of mushroom mycelium as a means of ecological restoration is a new and exciting concept with profound implications.

And of course, we have to become true conservatives, learning to conserve energy in every and any way possible. Gains in conservation and efficiency can be achieved through technological innovation (Third World sawmills, as Imhoff et al. point out, are grossly inefficient), but also through conscientious living. Time to roll up our sleeves folks, weve got a lot of work to do.